The newest COVID vaccine does not have more side effects than previous versions. If anything, side effects have trended downward over successive doses and formulations. CDC surveillance data from the 2023-2024 season found that rates of local and systemic reactions were similar to the prior season, and longer-term tracking shows a steady decline in overall reactogenicity since the original shots.
Side Effects Have Decreased Over Time
A longitudinal study tracking reactions from the original primary series through the JN.1-updated vaccine found that total symptom scores dropped consistently with each new dose. Severe reactions fell from 26.2% after the first original dose down to 3.3% after the JN.1 update. This decline was most pronounced in people under 45, whose immune systems tend to mount stronger (and more uncomfortable) responses. People 45 and older had consistently low reactogenicity throughout, so their experience hasn’t changed much either way.
This pattern makes biological sense. Your immune system has already encountered the spike protein multiple times through previous vaccination or infection, so it doesn’t need to mount as aggressive an initial inflammatory response to build protection.
What Side Effects to Expect
The most common reactions are the same ones reported since the original rollout: soreness at the injection site, fatigue, headache, muscle aches, and sometimes a low fever or chills. These typically start within hours of vaccination and resolve within a few days. Some people get no noticeable side effects at all.
If you’ve had a COVID vaccine before, your experience this time will likely be similar or milder than what you remember. The type of reactions hasn’t changed with the updated formulas, only the frequency and intensity, both of which have gone down.
Pfizer and Moderna vs. Novavax
There are meaningful differences between the mRNA vaccines (Pfizer and Moderna) and the protein-based Novavax option. In a direct comparison among healthcare workers, Novavax recipients reported an average of 1.7 systemic symptoms compared to 2.8 for Pfizer recipients. Only 24.2% of Novavax recipients had a moderate or higher-grade symptom, versus 43.8% of those who got Pfizer. Novavax recipients also reported 12.5% fewer local reactions like injection-site pain.
The productivity impact was notably different too. Novavax recipients averaged 0.7 hours of missed work from side effects compared to 1.4 hours for Pfizer. Hours of reduced productivity were 0.8 versus 2.4. If you’ve had rough reactions to mRNA vaccines in the past, the protein-based option may be worth asking about.
Myocarditis Risk With Updated Formulas
Heart inflammation (myocarditis) has been the most closely watched serious side effect since early in the vaccine rollout, particularly in adolescent and young adult males. The risk has dropped substantially with each generation of the vaccine. Early in the pandemic, rates in the highest-risk group (young men after their second Moderna dose) were roughly 1 in 3,000. By the first booster in fall 2021, that fell to about 1 in 10,000. For the 2023-2024 formulation, the best estimate in the highest-risk group is approximately 1 in 37,000.
When myocarditis does occur after vaccination, it typically appears within seven days. Symptoms include chest pain, shortness of breath, or a sensation of a racing or pounding heart. Most people who developed vaccine-related myocarditis recovered after medication and rest. Booster doses consistently carry lower myocarditis risk than primary series doses did.
Who Tends to Feel It More
Younger adults generally experience more noticeable side effects than older adults. This has held true across every version of the vaccine. People under 45 report higher symptom scores, though as noted above, even their reactions have diminished with each successive formulation. Older adults tend to have mild reactions regardless of which dose they’re on.
Males under 30 remain the demographic most likely to experience myocarditis, though the absolute risk is small and has decreased with each updated formula. For children aged 6 months through 11 years, vaccine doses are lower than adult doses, and severe reactogenicity data in this group is limited but has not raised new safety signals.
Managing Side Effects
Most side effects need nothing more than time. If soreness, fatigue, or a mild fever bothers you, over-the-counter pain relievers like ibuprofen or acetaminophen can help after you’ve received your shot. Staying hydrated and resting for a day is usually enough. The vast majority of reactions clear up within one to three days.
If you notice chest pain, shortness of breath, or heart palpitations within a week of vaccination, that warrants prompt medical attention to rule out myocarditis. An allergic reaction, which is very rare, would typically show up within minutes to hours and involves symptoms like hives, swelling, or difficulty breathing.